It looks like the dinosaurs were in decline for several million years before the impact event at Chixulub. As best we can tell the extinction of the large therapod and sauropod dinosaurs happened instantaneously, geologically speaking. That might mean days, months or even decades or more in reality; rock preservation in most places does not work at human timescale resolutions. Ir could be as long as centuries or millennia. I would personally put my money on decade to century scale; with only pockets surviving the first year, and perhaps some isolated pockets lasting over 1000 years (similar to mammoth post ice age). Edit: turns out my dinosaur knowledge was a few years out of date. Strike out the first sentence. Edit2: Following the OP edit, u/stringoflights adds the following and asked me to add it here for reference: Birds didn’t just evolve from dinosaurs, they are dinosaurs, and dinosaurs did not go extinct. We have more dinosaur species alive today than mammal species. It’s fine to pose a question about non-avian dinosaurs, but the fact that birds are dinosaurs is important when we are studying patterns of extinction. The bird “evolving back” from extinction that is mentioned in the edit is an example of iterative evolution. It is absolutely not a reason to ignore an entire radiation of dinosaurs, and it’s pretty important that that misconception is corrected. Iterative evolution means that the same or similar structures arose from the same source population at different times. It doesn’t mean the wholesale evolution of the same species twice. In the case of the Aldabra rail, which is what the edit mentions, the white-throated rail has colonized the island from nearby and evolved flightlessness more than once. More on reddit.com

There are a few answers to this question. Number 1: They didn't! Birds are one group of the dinosaur family tree , and they survived to today. Number 2: All sorts of ways. Dinosaurs first evolved about 230 million years ago, and ever since, they have been gradually evolving new forms and losing old ones to extinction. Most species probably didn't persist for more than a few million years at a time. Changes in food supply, climate, sea level, diseases, etc. etc. can all contribute to extinction. Number 3. The big one, and the answer you're probably looking for: At the very end of the Cretaceous Period, all dinosaurs aside from a handful of birds disappear from the fossil record (along with a good 70-80% of everything else). At this time, an asteroid struck the earth. This is understood very well . Sure, it caused a big explosion, tsunamis, and raining debris, but that's not what caused the extinction. The soot and ash from the explosion would have spread across the planet and lingered for a while, which appears to have had three major effects: 1) reducing sunlight, which is bad for plants, 2) reflecting heat and causing cooler global temperatures, and 3) after the reflective "cooling" particles dispersed, the leftover greenhouse gases would have led to warmer climates for a while. All of these effects contributed to ecosystem collapse, in the water and on land. Now, there is some debate as to whether or not the asteroid impact was the only factor. Around the same time, there was an enormous amount of volcanic activity going on in India, over many thousands of years, spewing out gases that may have had similar effects to the asteroid-dust I described above. It's still unclear how much of an impact this had on global ecosystems. Also of note: there is some evidence that ecosystems toward the end of the Cretaceous were being stressed by changes such as rising sea levels. This probably wouldn't cause a mass extinction by itself, but certainly may have made ecosystems more vulnerable when the big effects came. So what happened? An asteroid hit and caused ecosystem collapse around the globe. There was a preposterous amount of volcanic activity in India which may have contributed as well. And ecosystems might have been vulnerable ahead of time due to other global changes. A recent technical overview can be found here. More on reddit.com

Another question. If the impact was so powerful, do we have any idea where it happened? It should leave some marks.

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The event that you are referring to is usually called the KT extinction.

The current consensus among those who study the period is that there was a roughly 10km meteor that impacted Mexico, causing massive fires and a corresponding "nuclear" winter that made it difficult for life to survive in the sea and on the surface.

Many species went extinct as a result of this, including non-avian dinosaurs. The dinosaurs that eventually became modern birds did not die, nor did many other species.

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